As is known, many electronic devices are temperature sensitive, (i.e., must be operated in a given temperature range to ensure proper operation). Accordingly, such electronic devices typically include a temperature sensing circuit. If the electronic device includes integrated circuits, a portion of the temperature sensing circuit is on-chip. FIG. 1 illustrates such an on-chip temperature sensing circuit. As shown, FIG. 1 provides a temperature sensing circuit 10 located within an integrated circuit 12. Thermistor 20 is thermally coupled to the device under temperature test 22. Resistor R1 and thermistor 20 form a voltage-dividing network. Voltage Vtemp across thermistor 20 is sensed by an Analog to digital (ADC) 24. ADC 24 generates a digital valve of the sensed voltage. Processing module 26 uses algorithms 28 to convert the digital valve of ADC 24 into an actual temperature reading.
This arrangement has a number of disadvantages. The resistor divider approach requires a extra resistor, has poor power supply rejection, and lacks the ability to dynamically alter the divider to account for the large impedance range of a typical thermistor. As this arrangement does not provide the ability to adjust the range of voltage Vtemp sensed by ADC 24, this arrangement lacks the ability to take full advantage of the sensing resolution of ADC 24.
The temperature sensing circuit of FIG. 1 measures the temperature of electronic device or components with limitations. Such limitations include a limited temperature sensitive range. This is particularly true when employing a thermistor that has a resistance value exponential with respect to temperature. When using the resistor divider method this exponential response usually restricts the resistor divider to be used only within a narrow temperature range. The voltage-dividing network in this case is optimized due to ADC dynamic range limits for a particular temperature to be sensed across thermistor 20. Thus, when not operating in the optimal range the resolution of the sensed temperature provided by the measured voltage Vtemp is severely limited. Additionally, power supply noise issues and component count further hamper this type of arrangement.
Therefore, a need exists for an on-chip temperature sensing circuit that has a wide temperature sensing range and reduced external part count.